emerge pulled in new version of firefox, 45.2.
before i head over to mozilla, i just need your conformation that this is not a gentoo problem.

Any video (local news, youtube, ...) if I click full screen, it does not go fulscreen, video playback stops, music goes on, and firefox starts being not very responsive. It does not take any more ram ( i think) than usual, but cpu goes up to 6ish % (8core 4ghz).
I can not switch between tabs until i hit Esc button (and wait a bit) than I can switch tabs, but it takes several minutes to get a response. If I close 'playing tab" firefox closes it, but music still goes on. Any other surfing becomes very slow...

I also noticed that TOR (same 42 firefox version) does not play fullscreen, in fact symptoms are the same. Tor bundle is downloaded binary, and firefox is compiled.

I also use suckless surf browser, there are no problems with fullscreen playback.